Crosslinked polyimide high temperature composites and resins are useful in the aerospace industry and other industries because they are lightweight, high strength, high temperature materials. They are made by heating a reaction mixture of polymerizable monomeric reactants (PMR) to form an oligomeric imide resin in a pretreatment step. The imide resin may be further heated to crosslink it into a thermosetting polyimide resin. If desired, the crosslinking can be done under pressure to consolidate the imide resin into a polyimide resin article of a desired shape.
The reaction mixture used to make the imide resin includes an aromatic diamine monomer, an end cap monomer, and an aromatic dianhydride monomer. The aromatic dianhydride monomer may be used as either the aromatic dianhydride, a diester-diacid of the aromatic dianhydride, or a tetra acid of the aromatic dianhydride.
The pretreatment step, which forms the imide resin, generates and removes volatile gases that would otherwise form internal voids and surface blisters in the finished articles. Voids and blisters are undesirable because they detract from the physical properties and thermo-oxidative stability of the consolidated polyimide materials. Some references, including U.S. Pat. No. 4,197,339 to Paul et al., teach that up to about 10 volume percent (vol. %) voids in the final product are acceptable.
A conventional pretreatment step includes heating a reaction mixture from room temperature to about 204.degree. C. (400.degree. F.). For example, the reaction mixture may first be heated to 93.degree. C. (200.degree. F.) for one hour, then heated to 149.degree. C. (300.degree. F.) for another hour, and finally heated to 204.degree. C. (400.degree. F.) for two additional hours. Heating the reaction mixture to these temperatures provides energy to activate imidization reactions that generate most of the volatiles without activating crosslinking reactions. Substantial crosslinking during pretreatment is undesirable because crosslinked materials are more difficult to consolidate than materials that are not crosslinked. Because crosslinking can begin at temperatures higher than 204.degree. C. (400.degree. F.) and is likely to occur at temperatures higher than about 250.degree. C. (482.degree. F.), conventional pretreatments are done at temperatures no higher than 204.degree. C. The pretreatment step is frequently done under a vacuum to facilitate removal of the volatiles. Following a conventional pretreatment, the imide resin is considered capable of consolidation and crosslinking with no further evolution of volatiles.
Experience has shown, however, that polyimide materials prepared by a conventional pretreatment method often do not consolidate properly because of additional volatiles generated during consolidation. As a result, polyimide materials made with a conventional pretreatment can have high void fractions, inadequate physical properties, poor thermooxidative stability, and may be prone to blistering. These problems could be prevented if a pretreatment method capable of removing more volatiles from the reaction mixture was available. Additionally, to eliminate the inconvenience of using a vacuum system, it would be desirable if any new pretreatment method could remove volatiles without the use of a vacuum.
Accordingly, what is needed in the industry is a method of pretreating polyimide reaction mixtures that is capable of removing more volatiles than conventional pretreatment methods without the need to use a vacuum.